Solar Panel Degradation in California: What 25 Years Actually Looks Like

The Degradation Number: 0.3% to 0.5% a Year

Degradation is the gradual loss of a solar panel's power output as it ages — not a malfunction, just materials science. Sunlight exposure, heat cycling, and moisture slowly reduce a cell's efficiency.

The commonly cited industry figure, drawn from field studies of crystalline-silicon panels compiled by researchers at the National Renewable Energy Laboratory (Jordan & Kurtz, 2012, updated 2016), puts the median decline at roughly 0.5% of original output per year, with most fielded systems landing between 0.3% and 0.8% depending on climate, racking, and panel quality.

California's climate — hot, sun-heavy, low-humidity in most inland markets — sits in a fairly favorable range for module longevity compared to hot-humid climates, though rooftop heat buildup on low-clearance installations can push individual systems toward the higher end of that band.

Treat 0.3–0.5% as a well-established industry range rather than a number your specific installer is contractually bound to. Your actual rate is whatever your panel's datasheet specifies, and it varies by manufacturer and technology (monocrystalline PERC, TOPCon, and older polycrystalline cells don't degrade identically).

If a salesperson tells you your specific panels degrade slower than 0.3% a year, ask for the manufacturer's datasheet — that's a checkable number, not a talking point.

Two Warranties, Not One: Product vs. Performance

Solar contracts bundle two separate promises that protect against different failures, and conflating them is where most confusion starts. The product (or materials) warranty covers manufacturing defects — a panel that cracks, delaminates, or fails outright — and typically runs 10 to 12 years, occasionally up to 25 on premium panel lines.

The performance warranty is a different promise entirely: it guarantees the panel will still produce at least a stated percentage of its original rated output at a given year, commonly structured as roughly 98% at year one followed by a slow linear decline to somewhere in the 83–88% range at year 25.

If your panels underperform that guaranteed curve, the manufacturer owes you a remedy — repair, replacement, or a prorated payout, per the contract language.

If they degrade faster than the literature-typical 0.3–0.5%/yr but still stay above the warranty's guaranteed floor, you have no claim; the performance warranty is a floor, not a promise that your panels will hit the median rate.

This exact two-tier structure — separate coverage lengths, separate remedies — is exactly what California's consumer protection framework tells installers to disclose (see the next section). Read your specific manufacturer's warranty document for the real percentages; the ranges above are typical industry structure, not your contract.

How to read the year-25 number on your own warranty

Your installer's proposal or your panel's warranty document states three numbers — find them before you sign.

As one real example, REC Group's N-Peak performance warranty guarantees "at least 98% of the nameplate power output" in year one, a decline of "no more than 0.5% for a period of 24 years" on the standard N-Peak line (0.25%/year on the newer N-Peak 2 line), landing at a guaranteed floor of "at least 86%" of nameplate output at year 25 for standard N-Peak, or 92% for N-Peak 2 (REC Group, N-Peak Warranty).

Three panels from three manufacturers will show three different numbers here — Qcells, Tesla, Panasonic and others each publish their own schedule — so treat REC's figures as an illustration of where to look on your own document, not as a number to expect.

Read the fine print, too: REC's warranty covers only "natural degradation" of the cell, glass, and internal connections, and excludes damage from improper installation, weather events, or third-party damage — the same product-vs-performance split described above.

What California Law Actually Guarantees — and Doesn't

California doesn't set a degradation standard by statute. What it does require is disclosure. The CPUC's California Solar Consumer Protection Guide directs installers to spell out panel and inverter warranty periods and coverage terms separately from the labor/installation warranty, and to disclose who bears replacement cost — and expected response time — if equipment fails after the warranty expires.

Business & Professions Code § 7169 requires a solar disclosure document, with cost, complaint contact information, and your cancellation right printed in boldface 16-point type on the contract's front page; degradation and performance-warranty detail fall under the statute's optional "supporting information" category, not the mandatory line items, so don't assume it's automatically in front of you — ask for it.

Separately, Business & Professions Code § 7159 gives you a 3-business-day right to cancel — extended to 5 business days for a "senior citizen," a term defined elsewhere in the Civil Code (§ 1689.5) as a buyer 65 years of age or older — and caps any down payment at $1,000 or 10% of contract price, whichever is less.

Here's the part that matters most for a 25-year promise: California's Song-Beverly Consumer Warranty Act (Civil Code § 1791.1) caps the state's own implied warranty of merchantability at one year (with a 60-day floor), extending only as far as an express warranty when that express warranty is itself reasonable.

Translation: a 25-year performance warranty is a private contractual promise from whoever issued it, not something California law independently guarantees for 25 years. If that company doesn't exist anymore, the number in your contract is only as good as who's left standing behind it.

The Math: How Small Percentages Compound Over 25 Years

A 0.2-percentage-point difference in annual degradation rate looks trivial written down — 0.3% versus 0.5% — but it compounds over a quarter century into a meaningfully different system.

Run the arithmetic (not a warranty guarantee, just linear projection off year-one output) at three rates commonly cited in the literature, and the spread between the low and high end of the 0.3–0.8% range works out to roughly 12 percentage points of retained output by year 25. See the table below.

The practical read: at the low end of the range, your system is still producing over 90% of its original output when a 25-year loan or lease term ends. At the high end, you're closer to 80%.

Neither number is a defect — both are within normal expected behavior for crystalline-silicon panels — but they change the economics of a system you're financing for two and a half decades, particularly if your savings estimate assumed flat output rather than a declining curve.

Ask your installer which rate your specific panel is rated for, and ask them to show you a year-25 production estimate, not just a year-one one.

The Inverter: The Part That Actually Gets Replaced

Panels are the part everyone worries about; the inverter is the part that's actually likely to need mid-contract replacement.

Inverters are electronic components with a shorter typical service life than the panels they're attached to, and industry practice commonly separates standard string-inverter warranties (often shorter, sometimes extendable at added cost) from microinverter warranties (commonly written closer to the panel's own 25-year term to match it).

No primary source publishes current inverter replacement prices, so this page gives no dollar figure. Ask your installer, in writing, before signing, (1) what warranty length applies to your specific inverter model, (2) whether it's a string inverter or microinverters, and (3) what replacement costs after that warranty expires.

A degrading panel loses a fraction of a percent a year; a failed inverter can take your entire system offline until it's swapped. Get the number in writing rather than relying on an industry-average figure quoted verbally.

Lease and PPA Economics: A Rising Payment Against a Falling Output

If you own your system outright, degradation just means slightly less free electricity each year — a slow curve against a fixed asset you already paid for. A lease or PPA is a different shape of risk, because you're not paying for a fixed asset; you're paying an escalating price for a de-escalating output.

California's Consumer Protection Guide notes that lease and PPA contracts commonly carry payment escalators in the 1–3% per year range, while the system underneath that payment is producing 0.3–0.5% less each year. Those two lines move in opposite directions for the entire contract term, and the gap between them — not either number alone — is the real cost story.

The same guide lists a minimum energy guarantee (compensation owed if the system underperforms) as one of the specific questions it tells consumers to ask a lease or PPA provider before signing — it's a feature to ask about, not one every provider is required to offer — and separately notes that terminating a lease early or transferring it at home sale can carry buyout costs running into the thousands of dollars.

PACE financing, separately, typically runs 10 to 30 years per the same source. Before signing a 20-to-25-year lease or PPA, ask for a year-by-year projection that shows both the escalating payment and the declining output side by side — not a single blended "average savings" figure that hides the crossover point where the two lines meet.

What Happens When the Company Behind Your Warranty Disappears

A performance warranty is a promise, and promises require someone solvent to keep them.

This isn't hypothetical in California's solar market right now: Freedom Forever filed Chapter 11 on April 15, 2026, and its case converted to Chapter 7 liquidation effective August 7, 2026 (U.S. Bankruptcy Court, D. Del., Case No. 26-10522) and the company is now being wound down under a court-appointed Chapter 7 trustee rather than continuing to operate.

Sunnova filed Chapter 11 in June 2025; under its confirmed reorganization plan, its lease, PPA, and service operations transferred to SunStrong Management, which now administers those legacy accounts. SunPower filed Chapter 11 in August 2024 and sold its remaining assets.

None of this means every homeowner with one of these companies' names on their paperwork is out of luck — the workmanship/installation warranty (the installer's promise) and the panel manufacturer's performance warranty (a separate company's promise) are legally distinct, and one can survive even when the other doesn't.

But it does mean the Song-Beverly point above isn't abstract: check your paperwork for which entity issued which warranty, confirm that entity is still operating, and don't assume a 25-year number on a page is worth 25 years just because it's printed there.